1. Field of the Invention
The invention relates to vehicular steering control apparatus and method for controlling the tire wheel steering amount in accordance with the steering wheel speed.
2. Description of the Related Art
A differential steering control for controlling the tire wheel steering amount in accordance with the steering wheel speed is disclosed in, for example, Japanese Patent Application Laid-open No. 2001-138936. In the differential steering control, an actual wheel steering amount β is controlled as described in an equation below if a driver turns the steering wheel by a steering angle of α at a steering speed of Vα. In the equation, k is a constant, and ks(v) is a function of the vehicle speed V, and is a gain in steering wheel speed.β=α·k+Vα·ks(v)
Through this control, the tire wheel steering amount β is increased if a driver quickly turns the steering wheel, for example, to avoid a danger. Therefore, danger avoidance can be achieved merely by a small turn of the steering wheel. Conversely, if a driver slowly turns the steering wheel, the assist angle that is added to the multiplication product of the steering wheel angle α and the constant k is small. Thus, necessary fine steering operation becomes possible.
When right and left wheels face different road surface conditions, the wheels may experience different friction coefficients μ (hereinafter, this situation may be referred to as “different-μ state”), so that the braking amount will differ between the right and left wheels. For example, if the friction coefficient μ of the right side road surface is greater than the friction coefficient μ of the left side road surface (i.e., the right side road surface provides greater resistance), braking may result in a grip of the right side wheel, so that the vehicle body may turn clockwise in a plan view. In such an event, the driver will likely try to correct the orientation of the vehicle body to a normal orientation by quickly turning the steering wheel to the left (i.e., in the direction opposite to the turning direction of the vehicle). Normally, the vehicle body can be turned more easily if the delay in the response of the vehicle body orientation to steering operation is a little. However, it is a general practice to avoid high-response steering setting for ordinary vehicle running because stability is considered more important for ordinary running.
During the different-μ state, an event similar to the above-described event related to braking can occur when tire wheels are driven. That is, if the friction coefficient μ of the right side road surface is greater than the friction coefficient μ of the left side road surface, the right side wheel grips during an initial period of the driving, so that the vehicle body turns counterclockwise. In this case, the driver tends to quickly turn the steering wheel to the right in order to normalize the orientation of the vehicle body.